In some networking environments, communication networks may be formed when multiple interoperable nodes communicating over a shared medium detect the existence of other nodes. One example of such a network is a network that operates in accordance to the Multimedia over Coax Alliance (“MoCA”) MAC/PHY Specification v. 1.0 or v 1.1. In such a network, nodes may function as “client” or “slave” nodes, or as “master”/“network controller”/“network coordinator” (“NC”) nodes. A network typically has a single NC node and any number of client nodes, and the NC node transmits beacons and other control information to manage the network.
MoCA networks may be used for networking over in-home coaxial cable using orthogonal frequency division multiplexing (OFDM) modulation of data. OFDM is a digital multi-carrier modulation method in which a frequency band corresponding to a carrier comprises a number of closely spaced orthogonal subcarriers that are used to transport data. Data is divided into separate streams for respective subcarriers. Each network node has a modulation profile that specifies the modulation used for subcarriers transmitted by that node. For example, in accordance with one modulation profile, a first subcarrier employs 16-QAM (quadrature amplitude modulation with 16 constellation points, each representing one of the possible values of a four binary digit information word). A second subcarrier employs a denser modulation, such as 64-QAM (having 64 possible constellation points, each representing one of the 64 possible values of a 6 bit information word, thus, transmitting more information bits on the second subcarrier in the same amount of time). Each of the other subcarriers have a particular modulation density which may be greater, lesser or the same as the first and second subcarriers). Accordingly, other types of modulation, e.g., binary phase shift keying (BPSK) or quadrature phase shift keying (QPSK), can be used as well. The denser a modulation profile is (i.e., the more constellation points, and thus the more bits the group of subcarriers is capable of transmitting in the same amount of time as a result of particular modulation on the subcarriers), the less robust communication is for the associated node. That is, transmission using a more dense modulation scheme will be more susceptible to noise and other factors in the channel that can cause the packet error rate to be higher.
FIG. 1 is a diagram of a communications channel. A channel 110 may be a component of an RF frequency spectrum. Channel 110 may have a bandwidth of 50 MHz and may be centered at 575 MHz, although other bandwidths and center frequencies are available as well. OFDM transmits and receives data on subcarriers 120-1, 120-2, 120-3, 120-4, . . . , 120-N (generally 120) within channel 110; MoCA provides N=224, i.e., 224 subcarriers. Subcarriers 120 may be spaced 195 kHz apart in accordance with MoCA.
Interference 130 may be present on channel 110 and may arise from various sources, e.g., television signals such as Advanced Television Systems Committee (ATSC) digital television broadcasting signals. An ATSC interference signal has 6 MHz bandwidth and may have various center frequencies. FIG. 1 shows interference 130, e.g., ATSC interference, occupying bandwidth in channel 110 corresponding to subcarriers 120-3 and 120-4. As used herein, the term “sustained interference” refers to interference that is substantially non-varying and continually present. Interference 130 may also be dynamic interference, as opposed to “sustained interference” and may be caused by turning on a television station transmitter, for example. Messages (e.g., packets) that are transmitted using OFDM with subcarriers 120-3 or 120-4 may be affected by interference 130 and may result in errors.